Whether listening to a portable media player while traveling, or to a stereo or theater system at home, consumers often choose headphones. Headphones typically include a pair of earcups which encircle the user's ears and are held together by a headband. Headphones can be classified into two general categories based on the design of the earcups, namely closed-back or open-back earcups. Closed-back earcups surround the user's ears and have a sealed back. Open-back earcups also surround the user's ears but have a back which is open to the ambient environment surrounding the earcup.
Both the closed-back and the open-back designs have their own acoustic advantages and disadvantages. Representatively, closed-back earcups have good sound isolation since they are sealed off from ambient noise. In addition, the size and clamp force of the earcups can also be modified to further increase sound isolation. Features of the closed-back design, such as the sealed back, size and clamp force of the earcups allow this design to mechanically or passively attenuate any ambient noise. In some cases, however, closed-back earcups can also make use of an electronic active noise control (ANC) system for additional sound isolation. An ANC system is a noise cancellation system which can attenuate or cancel noise within the earcup by emitting an “antinoise” signal, which is an audio signal having, in theory, the same amplitude and opposite phase to that of the noise such that they cancel each other out.
Due to the closed design of closed-back earcups, however, they have stronger resonances. For example, standing waves can accumulate in the earcups. These standing waves can degrade sound quality and reduce the feeling of openness, which is often desired by a user. In addition, in a quiet environment, residual noise from electrical components within the earcup (e.g., a driver or microphone within the earcup housing) may be heard by the user.
Open-back earcups, on the other hand, have good sound quality due to their low resonances, feel more open to the user, and allow ambient noises to be used to mask some of the residual noises which would otherwise be heard by the user. Open-back earcups, however, cannot be used in noisy environments because their passive attenuation is by definition poor. In addition, since open-back earcups are substantially open to the ambient environment, ANC systems may not be able to efficiently cancel the ambient noise entering the earcup through the open back.